Out of curiosity, which of the engineering problems posed by the Starship/Superheavy system are the biggest potential showstoppers? My instinct is the belly flo

Out of curiosity, which of the engineering problems posed by the Starship/Superheavy system are the biggest potential showstoppers? My instinct is the belly flop/heat shield required to make Starship reusable, but I know nothing about rocketry.